2020 Fiscal Year Research-status Report
Electron Scattering and Electron Angular Anisotropy in Water and Aqueous Solution
| Project/Area Number |
20K15229
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| Research Institution | Kyoto University |
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Principal Investigator |
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| Project Period (FY) |
2020-04-01 – 2023-03-31
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| Keywords | electron spectroscopy / electron scattering / liquid water / photoelectron anisotropy |
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| Outline of Annual Research Achievements |
The project aims to uncover detailed electron scattering mechanisms in liquid water and aqueous solutions using photoelectron spectroscopy in international collaboration. Electron scattering is crucial for energy deposition, radical formation and biological damage by slow and fast electrons. Furthermore, a large spectral contribution is from scattered electrons, which needs to be quantified for a correct interpretation of spectra.
Low-energy electrons from aqueous solution have been mapped for the first time down to the ionization threshold, which revealed that peak features are distorted and diminish below a distinct threshold of 10-13 eV. This is true for both solvent and solute features. At the same time, scattering signal contribution increases disproportionally at these low energies, which cannot be explained by known scattering pathways. This hints at so-far undiscovered scattering processes at very low energies, which will be investigated further.
Secondly, the first ever study of water's vertical ionization energy over a very wide range of energies (from the threshold to ~1000 eV) demonstrated that the peak distortion at low energies (<13 eV) makes it impossible to accurately extract ionization energies, which has large implications for laser-bases studies on aqueous solutions.
Finally, the very first study on in-vacu liquid sheets have revealed that electron scattering gets altered by the increased local vapor pressure above the sheets surface. Further investigation will provide crucial details for studying the photoelectron angular dependence from such liquid sheets.
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| Current Status of Research Progress |
Current Status of Research Progress
1: Research has progressed more than it was originally planned.
Reason
Despite the ongoing corona pandemic, which unfortunately started at the beginning of the project, the study is proceeding very well thanks to the already established international collaboration. Measurements were done by partners in Germany while discussing the necessary steps online. Parallel data analysis and steering of the project was done from Japan. While not optimal, the online collaboration will continue until international travel is possible again.
Several measurement series at European synchrotrons already enabled the mapping of the electron scattering in liquid water from threshold energies to the X-ray regime for the first time. This revealed a very different scattering behavior at low energies with hints of so-far unknown electron scattering pathways.
At the same time a new and much more robust energy-referencing scheme for determining accurate ionization energies from liquid water was developed. This helped to reveal the impact of scattering on the apparent ionization energy extracted from low-energy measurements. Initial studies of aqueous solutions have been already started in parallel.
Furthermore, preliminary angular-dependent measurements from a liquid sheet have been conducted which promise to give insight into the role of water's (or that of a solution's) surface on electron scattering. The flat, rotatable liquid surface will be crucial to study, for example, the orientation or structure of molecular species at the surface of an aqueous solution.
Overall, the project is very much on track and already achieved preliminary results of milestones set for 2021.
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| Strategy for Future Research Activity |
For the time being, the collaboration will continue online until international travel is permitted again. The results of last year have shown that the project can proceed very well even under the current conditions.
As the next step, the study of ionization energies and electron scattering is extended to aqueous solutions with the preferable choice of surface-active species. This will give insight into role of the surface for both scattering and ionization-energy determination. By deliberately modifying the surface properties, insight into the surface escape barrier will be gained, i.e., the energy potential which the electrons have to overcome to escape into the vacuum. The barrier height, which is currently debated for the liquid state, is also an important parameter in scattering models, which also need to be refined to describe the newly found pronounced scattering behavior at very low energies in liquid water.
In parallel, angular-dependent measurements will be conducted for aqueous solutes to study the impact of electron scattering on the photoelectron angular distribution depending on the depth of the solute. In addition, the newly developed liquid-sheet technique provides an in-vacuo flat liquid surface and can be rotated to measure the photoelectron signal depending on the tilt angle. This varies both the probing depth an the relative orientation of aligned molecules relative to the detection direction, which can be utilized to study the surface orientation of molecules.
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| Causes of Carryover |
The project proposed international collaboration with mainly business travel expenses to join experiments in Europe and discuss with collaboration partners directly. This has unfortunately been made impossible by the ongoing corona pandemic. Thanks to effective online collaboration the research project is not negatively impacted by this. Travel will be resumed for a more effective collaboration as soon as the travel restrictions have been lifted. If possible the expenses will be used for 2-3 business trips to Europe and travel to conferences as originally planned in the proposal.
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